System and process for joining dissimilar materials and solid-state interlocking joint with intermetallic interface formed thereby

ABSTRACT

A method for connecting two dissimilar materials having different melting points is described wherein a the materials are heated together to obtain plasticization of the lower melting point material within a prefigured geometry within a first material in such a way so as to form intermetallic features within a solid state joint.

PRIORITY

This invention claims priority from and incorporates by referenceprovisional patent application No. 62/393,409 entitled System AndProcess For Joining Dissimilar Materials And Solid-State InterlockingJoint With Intermetallic Interface Formed Thereby filed Sep. 12, 2016.It also incorporates provisional patent application No. 62/533,851entitled The Joining Of Dissimilar Metals Through Formation Of DovetailExtrusions With Metallurgically Bonded Interfaces filed: Jul. 18, 2017.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY-SPONSOREDRESEARCH AND DEVELOPMENT

This invention was made with Government support under ContractDE-AC0576RL01830 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION Field of the Invention

The invention generally relates to methods for joining dissimilarmaterials and more particularly to connections between dissimilar metalshaving different melting points.

A world of rising energy necessitates approaches for reducing the amountof energy needed to perform standard tasks. Among approaches underdevelopment are lighter more fuel efficient vehicles. Reducing theweight of vehicles can be accomplished in a variety of ways includingreplacing heavier steel regions with lighter weight materials suchaluminum, plastic, carbon fiber or other dissimilar materials. However,difficulty has arisen in attempting to find ways to robustly joindissimilar materials in a way that provides the needed strength andresiliency that exists in structures that are made from the samematerial. Preferably, and in some instances by requirement, these seamsand interconnects must be welded together. Welding is fairly straightforward when the two materials have similar melting points but becomesmore and more difficult when the materials have vastly different meltingpoints or other characteristics.

Joining materials such as steel to aluminum, titanium, magnesium, orcopper, or any combination thereof, has proved difficult for a varietyof reasons. The prior art generally teaches that when these materialsare joined that the temperatures must be maintained generally low so asto prevent the formation of brittle intermetallic compounds, which aregenerally believed to cause the welds to be brittle and fail. Most priorart methodologies for joining dissimilar materials have focused ongetting rid of these brittle intermetallic portions especially when theintermetallic is the only means of joining the two dissimilar metalstogether.

One of the ways that this is done is by isolating the other metal fromthe molten aluminum during the arc welding process. Techniques such ascoatings, or inserting bimetallic inserts that contain portions of eachof the two types of metals and which were formed by another process andwelding the materials to the inserts are methodologies that have beentaught and practiced. However, the needs for these additional stepsincrease the complexity and cost and are generally unsuitable in a highthroughput manufacturing environment because of these issues andconcerns.

Hence what is needed is a process for forming high strength jointsbetween dissimilar materials in ways that are simpler cheaper and moreeffective than the current methodologies. The present invention is asignificant step forward in addressing these needs.

Additional advantages and novel features of the present invention willbe set forth as follows and will be readily apparent from thedescriptions and demonstrations set forth herein. Accordingly, thefollowing descriptions of the present invention should be seen asillustrative of the invention and not as limiting in any way.

SUMMARY

In one embodiment of the disclosure a method for connecting twodissimilar materials having different melting points is describedwherein a first material having a higher melting point than a secondmaterial is plasticized to fill a preformed groove, shape or depressionin the surface of a second material. The first and second materials areheated together (preferably rubbed and heated by friction) to obtainplasticization of the lower melting point material so as to cause theplasticization of the material and the movement of the material into thesurface feature (groove) in such a way so as to form intermetallicfeatures of the material within the solid state joint. Preferably and insome embodiments the temperature within the joint is controlled so as toprevent overheating of the weld. Examples of how this temperaturecontrol is achieved is described in more detail in the detaileddescription.

In some embodiments the method maybe performed using a friction stirwelding device that extends to a plunge depth greater than the thicknessof the second material. Various other features of the friction stirmethod may be appropriately modified so as to obtain the desired result.This may include varying the rate of traverse, process temperature,force pressures, rotation speeds, tool operational orientation, tip andshoulder temperatures, pretreatments including surface coatings,pre-fillings and other pretreatments and other parameters. In addition,various configurations and operations of the various apertures,features, grooves, dovetail shaped depressions or other features of thedevices may also be employed.

In one exemplary arrangement the groove contains nested dovetail groovesand the friction stir welding tool is plunged into to the lower of twonested dovetail grooves such that a portion of the material defining thelower groove contacts the friction stir welding tool and results in theforming at least one feature of higher melting temperature material thatextend upward into the lower melting temperature material. In additionto this single exemplary embodiment a variety of other embodiments arealso described and set forward.

The result of the implementation of this methodology for joiningmaterials is the formation of a joint that has a geometric shape definedby a preformed groove in a first metal material having a first meltingpoint that has been filled with a second material that has a secondlower melting point that has been plasticized and heated to both fillthe preformed groove and form intermetallic containing features. Thismethod and these joints can be found in a variety of heterogeneouscombinations including combinations of aluminum to steel and othermetallic and non-metallic combinations.

Various advantages and novel features of the present invention aredescribed herein and will become further readily apparent to thoseskilled in this art from the following detailed description. In thepreceding and following descriptions we have shown and described onlythe preferred embodiment of the invention, by way of illustration of thebest mode contemplated for carrying out the invention. As will berealized, the invention is capable of modification in various respectswithout departing from the invention. Accordingly, the drawings anddescription of the preferred embodiment set forth hereafter are to beregarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art configuration of a friction stir weldingarrangement for use in connecting different materials

FIG. 2 shows failed joints created by the arrangement shown in FIG. 1with no metalurgically bonded interlayer.

FIG. 3 shows an example of two materials of differing melting pointsjoined in an overheating process where intermetallics are intentionallyformed at the dissimilar interface.

FIG. 4 shows an example of one embodiment of the present disclosure.

FIG. 5 shows an intermetallic reinforced connection prior to tensiletesting.

FIG. 6 shows specimens after tensile testing when an intermetallic isintentionally formed at the dissimilar interface performed on variousdisclosed examples.

FIG. 7 shows plots of the data reflected in Table 1.

FIG. 8 shows SEM photographs of the intermetallic features in the filleddovetail sections corresponding to FIG. 7 and Table 1.

FIGS. 9-11 show various embodiments and configurations of friction stirwelding tools with mechanical contact shown being necessary to createmetallurgical bond.

FIGS. 12-14 shows various feature designs and the respective jointsformed therein.

FIGS. 15-18 show examples of such friction stir tooling.

FIG. 19 shows information of one set of process parameters.

FIG. 20 shows load vs. extension curves for different plunge depths.

FIG. 21 shows the failure morphologies discovered during tensile testinghaving the data shown in FIG. 20.

FIG. 22 shows different interlayer thicknesses that are generated underthe present embodiment.

FIG. 23 shows an arrangement of one tested embodiment

FIG. 24 shows a cross section of one tested configuration

FIG. 25 shows various dovetail geometries.

FIGS. 26-27 show the results of testing on the dovetail geometries ofFIG. 25.

FIG. 28 shows the maximum tensile load per unit length of weld (i.e.specimen thickness) plotted against different dovetail grooves andwelding conditions.

FIG. 29 is a plot of a function of extension for different dovetailgeometries.

DETAILED DESCRIPTION OF THE INVENTION

The following description includes examples of various embodiments ofthe present invention. It will be clear from this description of theinvention that the invention is not limited to these illustratedembodiments but that the invention also includes a variety ofmodifications and embodiments thereto. Therefore the present descriptionshould be seen as illustrative and not limiting. There is no intentionin the specification to limit the invention to the specific formdisclosed, but, on the contrary, the invention is to cover allmodifications, alternative constructions, and equivalents falling withinthe spirit and scope of the invention as defined in the claims.

The present invention centers around the joining of dissimilar materialsby utilizing a combination of embedded portions of a first materialwithin a preformed geometric shape or groove located in another materialunder process conditions and tooling geometries able to form anintermetallic interconnection or layer at the dissimilar interfacewithin the preformed shape or groove. Joining metals with differentmelting temperatures can be accomplished by extruding a lower meltingtemperature material into groves in a higher melting temperaturematerial while simultaneously forming a metallurgical bond within thegroove at the interface between the dissimilar metals. Joints with thisconfiguration exhibit superior strength and ductility compared to otherknown techniques for Friction Stir Welding (FSW) of aluminum to steel.

In one embodiment, a method for creating such a connection using afriction stir welding tool to heat the materials, cause plasticizationand the formation of intermetallic features and layers are described.Contrary to prior art which teaches that intermetallics and layersshould not be created within preformed grooves, the method describedherein teaches that creating intermetallics and layers within preformedgrooves significantly improve strength and ductility.

FIGS. 3-25 demonstrate various examples and embodiments of theinvention. Referring however, first to FIG. 1, a prior art configurationof a friction stir welding arrangement for use in connecting differentmaterials is shown. In such an arrangement a friction stir welding tool(FSW) and a material are brought into contact and the material(typically the lower melting point material) is plasticized by therotating tool. The tool and the plasticized zone that the rotating toolforms (stir zone) are traversed over a joint or along or raster path.When the lower melting temperature material is heated by the frictionstir welding device the lower temperature materials is plasticized andflows down into the preformed grooves in the higher temperaturematerial.

Typically the configuration is arranged such that the tool does notenter into the dovetail and is far from contacting the higher meltingtemperature material to prevent mixing conditions and elevatedtemperatures which would form intermetallic layers at the interfacebetween the higher and lower melting point materials. Generallyspeaking, it is believed that lower temperature welds are strongerbecause of the more finely grained microstructures that performing weldsunder these conditions can create. Therefore existing teachings in theart of friction stir welding try to run the weld as cold as possible andto avoid higher temperature operating conditions and the formation ofintermetallic interfaces. As a result the connections that are formed byplasticizing and pushing the softened material down into without formingan intermetallic connection or layer results in a purely mechanicalinterconnection that may provide mechanical strength in one directionbut does not include metallurgically bonded interlayer in otherdirection that the present invention provides. An example of the failureafter tensile testing is shown in the photograph in FIG. 2 for a singleand double dovetail joints with no metallurgically bonded interlayer.The lower melting point material, aluminum in this case, easily tearsout from the groove within the higher melting point material (steel inthis case).

In embodiments of the present invention, such as the example shown inFIG. 3, two materials of differing melting points are joined in aprocess wherein what is typically termed as overheating of the jointoccurs and an intermetallic layer is formed within in the dovetailinterconnect and strengthens rather than weakens the connection betweenthe higher and lower temperature materials.

In one example of this process called Friction Stir Dovetailing (FSD) acustom designed friction stir welding pin extends into the preformedfeature (groove, slot, dovetail, or other depression of a predesignatedgeometry) and generates heat sufficient to both plasticize the lowermelting point material such that it flows into the preformed featurewhile also heating the higher melting point material through rubbing toa point whereby the filled feature contains intermetallic features (orlayer) at the joint interface. An example of such an arrangement forperforming this method is shown in FIG. 4. This methodology has shown tobe effective when the traverse rate is between 10 mm to 200 mm perminute, the process temperatures range from about 300° C. to about 600°C., the vertical force is between about 1,000 pounds to about 25,000pounds and the tool rpm between about 50 rpm to about 1000 rpm. Allpossible parameter combinations for all possible materials have not beenexamined and parameters outside the general ranges given may alsoproduce the desired intermetallic. Thus the ranges given should not beviewed as restrictive but are exemplary. In other embodimentscombinations of other heating methodologies may also be utilized wherebyinsertion of the tip of the FSW into the higher temperature material isnot always necessary to achieve the formation of intermetallic features.

Contrary to the teachings in the art, the formation of thisintermetallic connection between, for example, aluminum and steel withinthese locking sections significantly improves joint strength. Thisprocess is particularly applicable to thick section joints where noother practical solution currently exists. An intermetallic reinforcedconnection is shown in FIG. 5.

The effectiveness of FSD with an intermetallic layer for an AA6061 andRolled Homogeneous Armor joint is demonstrated through tensile test datawhich shows specimens failing in the processed aluminum rather than atthe joint interface. (see FIG. 6).

TABLE 1 Extension at 75% of Extension at Maximum Maximum Maximum 75%Maximum Weld Load Load Load Load Set N/mm mm N/mm mm A 560 ± 6  1.42 ±0.04 420 ± 6  2.57 ± 0.05 B 1175 ± 36  2.73 ± 0.26 881 ± 27 5.36 ± 0.32C 797 ± 25 1.49 ± 0.04 582 ± 18 2.24 ± 0.03

The data reflected in Table 1 is plotted in FIG. 7 and illustrates theeffect of the formation of an intermetallic interface. In samples A, nointermetallic interface was formed. In examples B and C theseintermetallic interfaces were formed to different degrees. As the datashows the inclusion of the interface in sample B increased the max.tensile load by 107% and extension at max. load increased by 92%. Insample C the max. tensile load was increased by 42% and extension atmax. load increased by 5%. The improvements attained with B (withintermetallic) compared to A (no intermetallic) are even larger whenconsidering the load and extension at 75% of maximum load where failureis conventionally defined. Contrary to the teachings of the prior art, aprocess that includes infilling with intermetallic formation has shownto be an effective process of joining and welding dissimilar materialsand does not weaken the weld as the prior art suggests. In thisdescribed arrangement the entire dissimilar interface within thedovetail can react to stresses in more than a mechanical interlock inone direction. With intermetallic present, failure has been driven intothe bulk material away from the joint; a highly desirable failure mode.

This arrangement prevents sheering of the angled lower temperature piecesuch as aluminum and dovetail pullout resulting in greatly improvedstrength of the joint. This results in lap sheer samples that fail inthe lower temperature material, not at the joint. The results show thatusing FSP or FSW to extrude a plasticized material into an existingfeature/s in a material of higher plasticization temperature with theintent to create a mechanical interlock where an intermetallic iscreated at the dissimilar material interface within the dovetail duringthe process is superior to joints where the intermetallic interconnectare not formed. SEM photographs of the intermetallic features in thefilled dovetail section are shown in FIG. 8. Table 2 below shows theprocess conditions which generated the aluminum-steel intermetallicdescribed in FIG. 8 leading to the data depicted in FIG. 7.

TABLE 2 Tool Avg. Avg. Avg. Plunge Shoulder Shoulder Rotational ForgeWeld Avg. WC TIP Depth Scroll Temperature Speed Force Power TemperatureWeld Set (mm) Numbers Degrees (C.) RPM kN kW Degrees C. A 15.22 3 470170 35 4.95 475 B 15.45 3 470 150 57 4.85 490 C 15.45 2 470 400 19 5.25570

In addition to the various examples provided herein, a variety of otheralterations or various variations to the basic concept are alsocontemplated, and various modifications to the process and processingparameters can be undertaken. In one embodiment of the presentinvention, the friction stir welding tool is inserted or oriented so asto contact the bottom or side of the groove and generate additional heatat these points of contact. This method generates heat at the interfacewhere it is needed to form the intermetallic and is not generated in thebulk material where overheating could degrade the properties. Thisrubbing between the tool and underlying steel exposes atomically cleansurfaces which facilitate formation of intermetallics. In otherembodiments, the groove or the dovetail may contain features that whenbrought into contact with the FSW tool cause this heating to take placeand enhance the formation of intermetallic features. In otherembodiments the shape of the FSW tool or tip may be modified so as toengage selected portions of the groove or the groove may be variouslyconfigured to engage with the FSW tool in a particular way. Examples ofvarious modifications are shown in FIGS. 9, 10, and 11.

FIG. 10 for example, shows an embodiment wherein the dimensions of thedovetails are proportioned to be generally shallow and small as comparedto the pin diameter of this tool. Because the dovetails are shallow andsmall compared to the diameter of the pin tip, the overheated areacreated by contact between the pin and high temperature material isgenerally larger compared to other arrangements and is sufficient togenerate a hot layer of material that can form a continuous layer ofintermetallic features above and within the dovetails. In otherembodiments of the invention induction heating is used to producelocalized heating at the interface. In other embodiments plasticizedmaterial is forced through narrow openings between the tool and thehigher melting temperature material within the dovetail to produce hightemperature while the material flows. This localized heating within thegaps will cause localized heating within the gaps allowing for formationof intermetallic layers in the openings.

In other embodiments of the invention, the formation of intermetallichooks of higher melting material are formed by running the tool withinthe dovetail while the tool is biased such that it contacts one or bothside of the dovetail joint and higher temperature material into a hookas shown in FIG. 11. This provides an advantage in that it increases thearea of intermetallic contact between the dissimilar materials andassists in forcing the materials together. In other embodimentsgenerally squared grooves are formed in the higher temperature materialand then heated with the friction stir welding tool to cause the cornersof the device to rise and form hooks in the lower melt temperaturematerials. In other embodiments the heating process forms intermetallichooks. These hooks are formed by plunging the tool into to the lower oftwo nested dovetails (as shown in FIG. 12) such that the edges of thetool contact the corners of the lower dovetail resulting in theformation of two hooks of higher melting temperature material thatextend upward into the lower melting temperature material.

While this specific example is provided the particular squared form ofthe groove should not be seen as limiting and it should be understoodthat various other embodiments wherein the geometry provides thatpushing the tool into a fabricated groove or slit or against the edge ofa groove slit so as to cause the higher melting temperature alloy toform a hook or other feature that extends into the lower meltingtemperature material during friction stir processing, welding ordovetailing are also contemplated. Examples of such configurations andembodiments are found for example in FIG. 13.

In other arrangements such as the one shown in FIG. 14 mechanicalinterlocking is accomplished by deforming groves that are easier orfaster to manufacture. In instances a rastering grid can be produced.When the friction stir processing tool is sufficiently close to orcontacts these grooves during welding, these grooves can have sectionsthat are deformed and form intermetallic features that fill groves andprovide strong interlocking. While straight grooves are shown forpurposes of illustration this is not meant to be limiting. Variousalternatives and modifications can be undertaken to deform the grooveduring welding to create mechanical interlocking or increase the amountof mechanical interlocking. In addition to the geometry that is shown avariety of other geometries including nesting features, multiple T-slotsor notches or other fabricated features may be used to created layers ofinterlocking features. In some embodiments the dovetails or othermechanical interlocking features with rounded corners improve flow ofmaterial into the dovetail and reduce fatigue.

Preferably the tool temperature and force are maintained constant so asto provide consistency along the weld path and manage the strength ofthe various parts. This is accomplished in one set of embodiments bycontrolling the tool temperature control algorithm and a force controlalgorithm in conjunction with techniques where the tool contacts thedovetails. Constant tool temperature and position improves consistencyof the intermetallic layer and uniformity of size of generated hooks ornew features along the weld path and from part to part. In someapplications improved performance was obtained when a two piece frictionstir welding tool was utilized wherein the pin and shoulder of the toolcan move axially relative to one another.

In cases where the pin is contacting the high temperature materialwithin the dovetail, the pin can extend into the dovetail as material isworn from the pin without affecting the shoulder position. This could bedone for example by having a servo actuated pin and shoulder that allowsfor selective connection and release. In another embodiment a springloaded pin could be used to force more material out and keep pin lengthrelatively constant despite wear on the pit itself. In anotherembodiment of the invention the upper low melting temperature materialsare being extruded into the dovetail groves of underlying high meltingtemperature materials using a counter-clock wise threaded pin within theFSW tool. Thus clockwise rotation of the tool causes downward extrusionof the plasticized material. Locally heating dovetail interface causedmetallurgical bonding by kneading action.

In as much as the present invention utilizes the combination ofmechanical interlocking with intermetallic formation variousmodifications and alterations could be made so as to enhance and fosterthe development of intermetallic interconnects at a lower temperature.In one example, a materials such as Yttrium, Tungsten, Molybdenum, Ironcompounds and others could be applied to reduce the temperature orimprove the rate of formation of intermetallic to the dovetail jointsprior to FSD. This could be done using cold spray, thermal spray or anyother deposition method which can also be used to tailor the compositionof the intermetallic layer.

In another example pre-filled dovetails are utilized wherein themechanical grooves in the higher temperature material is pre-filled withlower melting temperature material. This can reduce or eliminate theexcess material that maybe removed from the top of the lower meltingtemperature material when filling the dovetail. This prefilling can beaccomplished by filling the groove with bar stock, powder chips of otherforms of the lower temperature material. In another embodiment alaminated approach could be used wherein arc welding, strip cladding orother fusion welding techniques are used to bond lower temperaturematerials such as aluminum inside of the dovetails and then executefriction stir welding to create the intermetallic hooks andinterconnects. This can improve process robustness, welding speed andcan prevent the formation of a recess at the top of the weld frommaterial lost to fill the dovetail.

In one application friction stir welding was used to apply cladding bycreating a dovetail grid similar to the grid shown in FIG. 14. While theterm grid connotes a square or rectangular geometry it is to beunderstood that the grid can be circular or any other shape and whilethe grid would likely be two dimensional on flat cladding and threedimensional on contoured cladding these parameters are not limiting.This cladding arrangement allows for the use of a grooved grid forforming a mechanical interlocks that will in turn dramatically improveresistance to ballistic impact and provides the multi directionalstrength and fatigue life of thick section cladding. In one set ofpreferred embodiments a two pass technique for accomplishing this wasutilized wherein one pass of the friction stir device was made to createthe intermetallic layer or layers along the dovetail interface and asecond pass, run at much cooler process conditions followed whichincreased the strength of the material inside the dovetail whilemaintaining the intermetallic interface.

Specialized tooling capable of 1) heating the dissimilar metal interfacewithin or adjacent to the dovetail to temperatures higher than the stirzone and 2) “kneading” a thin interfacial layer to locally mix thedissimilar metals can also assist in the performance of the method. Thesimultaneous localized temperature rise and kneading at the dissimilarmetal interface are achieved by pressing the tool against the highertemperature material during FSD. Tool and dovetails configurations canbe designed in coordination to allow for contact anywhere or everywherewithin the dovetail. This method enables the formation of intermetallicand/or amorphous bonding at the dissimilar interface, which reinforcesthe joint, while stir zone temperatures are kept low. A low stir zonetemperature are preferable for minimizing degradation of bulk materialproperties in the lower melting point material. Examples of such toolingare shown in FIGS. 15-16. Tooling Friction stir tools have beendeveloped with features specifically intended to extrude lower meltingpoint metal into dovetail grooves in a higher melting point material;while simultaneously forming a metallurgical bond at the dissimilarinterface. The tools contain an insert (such as tungsten-carbide,tungsten-rhenium, polycrystalline boron nitride, etc. . . . ) within thepin tip which enables high wear resistance and consistency of themetallurgical bond. For example, a tungsten-carbide insert could bepress fit into an H13 steel FSW tool. The insert rubs against the highermelting point material, within the dovetail groove, and givesdramatically improved tool life and wear resistance compared to toolswithout a tip insert. The intent is to protect insertion of high wearresistant materials into FSW tools as a pin, or pin insert, for thepurpose of rubbing the higher melting temperature in a dissimilardovetail joint—for the purpose of creating a metallurgical bond. Theseillustrative examples are not intended to restrict the possibleconfigurations.

In one embodiment a tip insert is the tool feature that interacts withthe dissimilar material interface. The insert can be flat or convex, andmay contain scrolls, stepped spirals or other features that enhance“kneading” of the dissimilar materials and also expose new material andpush surface impurities away from the interface. Illustrative insertconfigurations are shown in FIG. 17. The insert may be circular,hexagonal, square, or any shape desired. These features are unique fromother tip features attempted in FSW because these features are designedto push material outward and to encourage the formation of ametallurgical bond at a dissimilar metal interface.

FIG. 16a shows the H13 FSW tool with circular tungsten carbide tipinsert after eight linear feet of welding. The pin is not deformed andthe tungsten carbide insert has no visible sign of wear. By comparison,FIG. 16b shows a H13 FSW tool without a tip insert after eight linearinches of welding. Wear and deformation is immediate when a hardenedinsert is not used when rubbing to generated an intermetallic bondinglayer. Use of a tungsten carbide insert dramatically improves tool wearfor this new process. In one set of tests two examples of FSW toolshaving tungsten-carbide inserts within the pin tip were used, see FIG.18. The upper tool contains a cylindrical insert and the lower toolcontains a hexagonal insert. The cylindrical insert configuration wasused to join AA6061 to Rolled Homogenous Armor (RHA) MIL-DTL-12560J in alap weld configuration. The upper material of the lap joint was 0.5″thick AA60601 and the lower material was 0.5″ thick RHA containing asingle dovetail. A single tool was used to weld eight linear feetwithout visible signs of wear or degradation of the tip insert. FIG. 19shows that in use, the temperature was higher at the face of the tipinsert (area of rubbing on the RHA) than at the shoulder which is animportant for making the key feature for forming a metallurgical bond.In traditional FSW, the shoulder is the highest temperature—which is notdesirable in the present arrangement.

In one set of experiments nine sets of lap joints were welded having keyparameters within the following ranges. Tool speed 100-250 rpm, federateup to 7.5 cm/min, force 25-100 kN, torque 250-350 Nm, tip temperature450-550 degrees C., shoulder temp 400-500 degrees C. These samples werethen tested at different plunge depths. FIG. 20 shows load vs. extensioncurves for different plunge depths (0.599″, 0.603″, 0.608″ and 0.620″)of the FSW tool. The 0.599″ case did not involve rubbing of the tipinsert within the dovetail grooves for the express purpose ofdetermining baseline strength in the absence of a metallurgical bond.The other three plunge depths were intended to impart increasing amountsof rubbing between the tip insert and base of the RHA dovetail. A totalof 26 specimens were tensile tested (qty 6 for 0.599″, qty 5 for 0.603″,qty 6 for 0.608″, qty 9 for 0.620″). The four curves in the followingplot represent an average of each grouping. From this plot it is clearthat the highest strength and largest ductility (extension) is for aplunge depth of 0.608″. A smaller plunge depth of 0.603″ gives lowerstrength and ductility as does a larger plunge depth of 0.620″.

FIG. 21 shows that the failure morphology during tensile testing (AA6061being pulled to the right and RHA being pulled to the left) is verydifferent for each of the four curves in the above plot. For the 0.599″plunge depth, the aluminum simply pulls out of the dovetail as thealuminum corner plastically deforms. For the 0.603″ plunge depth, a weakmetallurgical bond is formed which fractures in a brittle manner andthen shears at the aluminum corner. For the 0.62″ case, themetallurgical bond does not fracture and failure occurs in the bulkaluminum within the dovetail resulting in higher strength and ductility.The case with the highest strength and ductility is for the 0.608″plunge depth where shear failure occurs in the bulk material.

FIG. 22 shows different interlayer thicknesses (2.2 micron on left, 1.3micron in middle and 100 nm on right) that are generated. The phase (forexample, intermetallic or amorphous) and strength of the metallurgicalbond at the dissimilar interface are affected by temperature as well asthe strength of the heat affected zone in the aluminum. Controllingtemperature in the stir zone and the dissimilar metal interfacesimultaneously can be performed by modulating the spindle axis speed,torque, current, power or any combination of these variables. Thetemperature of the dissimilar interface is preferably controlled bymodulating the position, forge force or motor torque of the forge axis.Control algorithms governing the temperatures in the stir zone and atthe dissimilar interface operate independently, but may be linkedtogether as part of multivariable control scheme.

In one embodiment the spindle axis is used to control the temperature ofthe stir zone and the forge axis to control the temperature at or nearto the dissimilar interface. This could be done with a monolithic toolor with a two piece tool where the shoulder and pin can move relative toeach other along the forge axis. Another embodiment of this concept isto use the spindle axis to control the temperature at or near thedissimilar interface and the forge axis to control the temperature ofthe stir zone. This could be done with a monolithic tool or with a twopiece tool where the shoulder and pin can move relative to each otheralong the forge axis. Typically the spindle axis is controlled bycommanding speed, torque or power to regulate temperature and the forgeaxis is controlled by commanding a force, velocity or position change toregulate temperature. In FSW machines that allow the pin to rotaterelative to the shoulder one spindle axis can control the temperature ofthe stir zone, while the other control the temperature at the dissimilarinterface.

The friction stir dovetailing process can also be used to joindissimilar materials with a myriad of different joint configurations.For example, metal with a higher melting point (for example steel) canbe “buttered” (coated) with a metal having a lower melting point (forexample aluminum) such that subsequent fusion welding can be performedto form previously impossible configurations for dissimilar metals. This“buttering” can be single or double sided and the thicker section can beeither the higher or lower melting point material. The buttered layer,or underlying steel, may contain features (not illustrated due to thelimitless embodiments) such as tabs, angles, holes, slots and otherfeatures that enable subsequent fusion welding of joints having a finalconfiguration that is otherwise unweldable for dissimilar metals.Buttering can also enable subsequent fusion welding of a nearlylimitless array of other structures and attachments such as extrusions,brackets, threaded shafts, fittings and so forth (also not illustratedhere due to the numerous possibilities). Buttering can also overcomeclearance/access issues during manufacturing that are currentlypreventing adoption of FSW in vehicle applications. The butteringapproach can also enable fusion welding in areas for materials wherewelded properties are more beneficial than FSW; all while simultaneouslyallowing a joint between dissimilar metals. Another example is theenabling of interior joints that are otherwise impossible for dissimilarmetals.

The chemistry of intermetallic or amorphous layers/regions affects themechanical properties and microstructure of the metallurgically bondedinterface. The intent is to protect the use of cold spray to deposit alayer of metal within the dovetail to modify the chemistry of themetallurgical bond at the dissimilar interface. One embodiment of thisconcept is to spray a thin layer of cold spray material on the innersurfaces prior to friction stir dovetailing. Alternatively, the dovetailgroove could be filled partially or fully with cold spray material priorto FSW. For example, cold spraying 7000 series aluminum into thedovetails of underlying steel would reduce/eliminate the presence ofaluminum alloying elements and therefore change the structure/propertiesof the bonded interlayer.

The following examples are provided as illustrations of the principlesand embodiments described above:

EXAMPLE 1

Solid-state joining of thick section aluminum to steel plate wasachieved using a custom designed pin tool in a friction stir weldingdevice to flow a lower melting point material (AA6061) into dovetailgrooves previously machined into the surface of an underlying materialhaving a higher melting point (rolled homogeneous armor [RHA]).Repeating dovetails form a mechanical interlocking structure akin tometallic Velcro, however the forming of intermetallic interconnects bythe friction stir welding tool strengthened this interconnection. In oneexample, 38.1 mm (1.5 in.) thick AA6061 was joined to 12.7 mm (0.5 in.)thick RHA plates. Tensile test data showed specimens failing in theprocessed aluminum rather than at the joint interface.

Plates of RHA procured to MIL-DTL-12560J were dual disc ground to athickness of 12.7 mm and pre-machined dovetail grooves shown in FIG. 23.The RHA plates were inserted into AA6061-T651 sandwich structures havinga total thickness of 38.1 mm. FSD was performed using a tool made fromH13 tool steel that was heat treated to obtain RHC 45. The one-piece FSWtool consists of a scrolled shoulder and a frustum shaped (6.1°)threaded+3 flatted pin. FSD was performed using a tool rotational speedof 275 RPM and welding speed ranging between 25-50 mm/min. All weldingwas performed using a position control mode where the forge force is aresponse variable of the commanded plunge depth. Welds were made on thetop side, then machined flat, and the assembly was turned over to weldthe bottom side. Tensile specimens were cut from the welded Al-steel toan average thickness of 12.0 mm using a water jet. A cross section isshown in FIG. 24. Standard grinding and polishing sequences werefollowed for metallographic sample preparation and final polishedsurface was obtained using colloidal silica (<0.05 μm).

A scanning electron microscope (SEM) equipped with energy dispersivespectroscopy (EDS) was employed to investigate the intermetallicformation. Tensile testing of sandwich plates was performed using a 50kip MTS test frame to ascertain tensile test and microstructuralobservations. The results of that testing are shown in FIGS. 26-27.Structural analysis of a dovetail joints between AA6061 and RHAsubjected to tensile load was simulated using LS DYNA finite elementsoftware. The simulation predicted the failure of tensile specimenswith, and without, the formation of IMCs along Al and RHA dovetailinterface. Cases for 1, 2 and 3 dovetails having the outlined geometries(shown in FIG. 25) were structurally analyzed.

From the finite element simulations, it was observed that shear failureof the Al dovetail occurred for configuration with one, two and threedovetails when no intermetallic connection is present. Therefore, simpledovetail interlock without bonding doesn't have impact on structuralintegrity regardless of the number of dovetails. The testing showed thatjoint strength is improved when IMC is present at the Al-RHA interfacewithin the dovetail. In the case of IMC being present, only two dovetailfeatures are required to cause failure in the bulk Al. In general, theresults of this structural analysis indicate that, the presence of IMCsformation improves joint efficiency in the FSD process. As a result,steps were taken to generate an IMC at the Al-RHA interface whilesimultaneously filling the dovetail grooves.

Transverse macro sections of Al-RHA joints with different dovetailgeometries are shown in FIG. 25. The macro-sections clearly demonstratethe effective filling of Al into the dovetail grooves regardless ofdovetail geometric variations. The FSD process is quite robust in termsbeing able to fully fill the grooves. For example, welds were performed(from 200 to 275 rpm and 25 to 100 mm/min) with the tip of the toolranging from 2 mm above the RHA surface to having the tool tip incontact with the bottom of the dovetails. In all cases, the grooves werefully filled with no voids observed. While FIG. 25 provides a macro-viewof weld cross sections in terms of defect formation and dovetailfilling, metallographic analysis is needed to determine the bondingstate along the Al-RHA interface. SEM analysis at the Al-RHA interfaceof specimens in are shown in FIGS. 26 and 27 respectively.

The data indicates that interfacial bonding has occurred due to theformation of an IMC measuring 0.5 μm to 1 μm thick in narrow dovetailgrooves and 1.0 μm to 2.0 μm thick in wider dovetail grooves. The SEMmicrographs suggest that incipient melting of AA6061 during FSD mightcause bonding between RHA and Al with the formation of an intermediatetransition layer which will be further confirmed as IMCs from energydispersive spectroscopy (EDS) analysis. The formation of IMCs wasconfirmed by elemental quantitative analysis using EDS. The spot (area)and line scanning energy spectrum results are combined with the SEMmicrograph in FIG. 27. The atomic percentage of corresponding line scansof Al and Fe at the intermediate transition layer indicate a diffusionprofile of Al and Fe across the interface suggesting IMC formation.Moreover, the EDS spot analysis of this layer showed 79 at. % Al and 14at. % Fe. In FSD, intense plastic deformation of AA6061 by the stirringtool might cause incipient melting of Al in close proximity to the RHAdue to high localized temperature. The increased heat input caused bythe tool contacting and deforming the RHA resulted in the formation ofpossible multiple IMCs (AlFe, Al3Fe, FeAl2, Al4Fe, Al13Fe4, Al5Fe2 etc.)at the bonding interface which might be further confirmed fromtemperature measurement during FSD, phase diagram analysis andcorresponding X-ray diffraction analysis.

The macro cross section shows the deformed layer of RHA near the upperregion of dovetails where the stir tool intentionally contacted the RHAduring processing to locally increase temperature and promote IMCformation. Consequently, the growing of IMCs were evident outside thedovetail in the SEM and EDS analysis. Frictional heating due to contactbetween the stir tool and RHA may result in the Al being melted locally,thereby resulting in the formation of IMCs. According to the EDS spectraand elemental composition, the intermetallic compounds FeAl2, Fe3Al orFe2Al might form in the Al-RHA interlayer.

FIG. 28 presents the maximum tensile load per unit length of weld (i.e.specimen thickness) plotted against different dovetail grooves andwelding conditions. It was observed that nested dovetails result inhigher strength than single wider dovetails regardless of welding speed.The higher load carrying capacity provided by nested dovetails is due tothe additional interlocking that resists deformation in the tensile andtransverse directions. In the absence of IMC, there does not appear tobe a statistical difference in the load at failure on the weld speedrange of 25-50 mm/min. However, inclusion of the IMC within the widersingle dovetail at 25 mm/min was found to increase strength compared tothe case of no IMC. This speaks to the role of IMC formation forimproving joint strength. The narrow dovetails welded at 25 mm/min haveIMC formation outside the dovetail and interestingly show higherstrength than the wider dovetails with IMC. From this data, we concludedthat the formation of IMCs significantly improves joint strength.

The normalized load (load per unit weld length) as a function ofextension for different dovetail geometries is plotted in FIG. 29.Failure of the narrower dovetail specimen (A) occurred due to fractureof the brittle intermetallic layer on one side of the sandwich structureat peak load which is followed by ductile failure of bulk Al due toeccentric loading. For the specimens D and E, successive separation ofdovetails occurred after reaching the maximum load as the dovetails tendto unzip one pair after another. This phenomenon is indicated by thechanges in slope of the load curves on their descending part as tensiletesting progress to joint failure. For the nested dovetail welded at 25mm/min corresponding to specimen C, failure occurred in the processed Alrather than at the dovetail interlock. As mentioned earlier the volumeof the filled Al in the nested dovetail is high enough to encounter thetensile loading near the region of the additional interlock, resultingin failure in the Al with the failure plane perpendicular to the loadingdirection. The failure of the tensile specimen C is similar to specimenB. However, an additional contribution of bonding between Al and RHAwith the formation of IMCs resulted in a bulk Al failure rather than afailure at the joint. This is indicative of the strength of the jointand demonstrates the viability of extending to a wide range of materialstack-up (50 mm or higher thickness) to form dovetail interlock.

While various preferred embodiments of the invention are shown anddescribed, it is to be distinctly understood that this invention is notlimited thereto but may be variously embodied to practice within thescope of the following claims. From the foregoing description, it willbe apparent that various changes may be made without departing from thespirit and scope of the invention as defined by the following claims.

What is claimed is:
 1. A method for connecting two dissimilar materialshaving different melting points comprising the steps of: placing a firstmaterial having a higher melting point than a second material, incontact with the second material within a groove and heating said firstand second materials to a temperature sufficient to plasticize saidfirst and second materials within the groove and form an intermetallicfeature therein.
 2. The method of claim 1 wherein said heating isobtained by friction.
 3. The method of claim 1 wherein said heating iscontrolled to prevent overheating.
 4. The method of claim 2 wherein theheating step is performed using a friction stir welding device thatextends beyond the thickness of the first material into the groove to aplunge depth greater than a thickness of the second material.
 5. Themethod of claim 4 wherein the intermetallic feature includes at leastone set of hooks.
 6. The method of claim 5 wherein the intermetallicfeature includes at least two sets of hooks.
 7. The method of claim 4wherein the friction stir welding tool is operated within the groove insuch a position so as to contact one or both sides of the groove andform the higher temperature material into a hook.
 8. The method of claim1 wherein said groove contains nested dovetail grooves and said frictionstir welding tool is plunged into to the lower of two nested dovetailgrooves such that a portion of the material defining the lower groovecontacts the friction stir welding tool and results in the forming atleast one feature of higher melting temperature material that extendupward into the lower melting temperature material.
 9. A jointcomprising: a geometric shape defined by a preformed groove in a firstmetal material that has been filled with a second material, the secondmaterial having a lower melting point than the first material, thesecond material having been plasticized and heated to both fill thepreformed groove and form intermetallic containing features at theinterface between the first and second materials.
 10. A method forjoining of aluminum to steel comprising the steps of: placing a piece ofsteel with a surface defining a dovetail groove therein in contact witha piece of aluminum having a thickness; and friction stir welding thealuminum and the steel with a friction stir welding tool that plunges toa depth greater than the thickness of the aluminum and enters thedovetail groove so as to heat both the steel and the aluminum to atemperature sufficient to plasticize and flow the aluminum into thedovetail groove and to form intermetallic features therein.
 11. Themethod of claim 10 wherein friction stir welding tool has a scrolledshoulder and a frustum shaped threaded and flatted pin.
 12. The methodof claim 11 wherein the friction stir welding is performed using a toolrotational speed between 50 and 5000 RPM and a welding speed rangingbetween 1-5000mm/min.
 13. The method of claim 12 wherein the frictionstir welding is performed using a tool rotational speed between 200 and400 RPM and welding speed ranging between 25-50 mm/min.
 14. The methodof claim 12 wherein the temperature of the shoulder of the friction stirwelding tool is less than the temperature of the tip of the frictionstir welding tool.
 15. The method of claim 12 wherein the groove is adouble dovetail groove.